1. Field of the Invention
The present invention relates to a speed change operation apparatus for a stepless speed changer device, configured to increase a rotational output in response to increase in rotational displacement of a trunnion shaft from a neutral position to an operational position.
2. Description of the Related Art
According to one type of speed change operation apparatus, the apparatus includes a pivotal body operably coupled to a trunnion shaft, a positioning body supported to be pivotable about a positioning axis parallel with a pivot axis of the pivotal body, and a cam mechanism for positioning the trunnion shaft at the neutral position. At the time of positioning, into a recessed portion of a cam (a constituent element of a cam mechanism) formed in one of the pivotal body and the positioning body, a cam follower (another constituent element of the cam mechanism) is engaged. And, the positioning body is pivotally urged by a spring so as to bring the cam and the cam follower into contact with other.
For instance, a speed change operation apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2009-48331 includes a first member as a pivotal body operably coupled to a trunnion shaft, a second member as a positioning body and a spring and an auxiliary spring that pivotally urge the second member. The spring is connected to the second member and pivots the second member with an urging force that progressively increases as the trunnion shaft reaches the higher speed side. The auxiliary spring is connected to a rod having one end thereof engaged in a cam hole formed in the second member. The rod is supported to be pivotable with the portion thereof opposite to the side engaged in the cam hole acting as the pivot. When the trunnion shaft is located at a neutral position or an area adjacent the neutral position, one terminal end of the rod contacts an inclined portion of the cam hole by the urging force of the auxiliary spring, thereby to press the second member toward the first member. When the trunnion shaft is located at a distant area more distant from the neutral position than the adjacent area, the one terminal end of the rod is pressed against an arcuate portion of the cam hole by the urging force of the auxiliary spring. That is, while the trunnion shaft is located in the adjacent area, the urging force of the auxiliary spring urges the trunnion shaft to the neutral position. Whereas, while the trunnion shaft is located at the distant area, the urging force of the auxiliary spring does not urge the trunnion shaft to the neutral position. With this speed change operation apparatus, even with appropriate setting of the urging force applied to the positioning member when the trunnion shaft is located at the neutral position and the urging force applied to the positioning member when the trunnion shaft is located at a highest speed position, the urging force of the spring becomes weaker as the trunnion shaft is located closer to the neutral position. Further, the urging force of the auxiliary spring is applied to the positioning member for only a portion in the operational range of the trunnion shaft adjacent the neutral. Because of these two respects, in the course of an operation of the trunnion shaft from one of the neutral position and the highest speed position to the other, there occurs change in the pressing force applied from the positioning member to the pivotal body, thus tending to invite change in the rotational resistance of the trunnion shaft. As a result, there arises a problem of variation occurring in the operational resistance.